What are the appropriate black iron and poly diameters for residential propane gas lines?

Since propane is higher pressure than natural gas, very small lines will work. But, it is required that you use sizes large enough for if natural gas becomes available in your area. This is how we do it here-- The main black pipe in the house is 3/4, with a couple inches protruding outside the wall. Each individual appliance is run with 1/2 inch black iron from the 3/4 main line. Outside from the tank to the house is usually installed by the propane co who owns the tank. For propane they often use flexible copper between the tank and home. You can use small 1/2 inch flexible copper for this. I know this sounds small, but it's fine for propane. Your interior black pipe is set for future natural gas if it comes some day by using the black pipe sizes I indicated. Don't forget to install drop pipes at the appliances to trap moisture or dirt.